In the show, beginning performances Tuesday at the Paramount Center Mainstage, Clarkson and partner Jeff Turner, 32, deliver a condensed, comedic take on all seven Potter novels in about 70 minutes. They aim jokes both straight at the kids in the audience and over their heads at the adults. There is also Quidditch.

Talking on his
phone from a Washington, D.C., sidewalk, Clarkson is more than willing to acknowledge their inspirations, especially the Reduced Shakespeare Company and its “The Complete Works of William Shakespeare (abridged).”

“You say inspiration, I say we just copied them,” he says, deadpan. “I was 15 when they came to London, and I remember being blown away by it. To watch them was amazing, and now to be compared to them is one of the greatest honors.

Clarkson was a struggling stand-up comic and commercial actor back in 2005 when a PR firm hired him on behalf of a London bookstore to entertain the crowd expected to line up to buy the sixth Potter book, “Harry Potter and the Half-Blood Prince.” He already knew the books, hooked after reading the first one aloud to his younger brother. The planned skit was a five-minute fast-forward through the saga so far, before the bookstore opened at midnight.

“I left the meeting and went away to find someone to play Harry Potter,” Clarkson says. “And I was walking through Covent Garden and [Turner] was there busking, doing a sort of comedy show.” They had mutual friends, but had never talked.

“I looked at him and sort of squinted my eyes and thought, if you kind of half look at him and half close your eyes, he kind of looks like Daniel Radcliffe a bit. We should talk,” Clarkson says with a laugh. “So we went and had a drink, and we got on. . . . And that was it. He was Harry Potter, which left me playing all 360 other characters in the show.”

Actually it’s more like 60 to 70 other characters, and he chose to divvy it up that way, thanks to a large collection of props accumulated in his stand-up career. “It’s literally me running around in a lot of different hats and wigs and props and glasses,” he says. “You wouldn’t believe it was just one actor.”

All Turner needed was some glasses and, to make things easy for everyone, the character’s name on his forehead in place of Potter’s famous lightning bolt scar. A hit outside the bookstore, they kept expanding the show and eventually moved on to Edinburgh Fringe Festival and toured around Great Britain. Now it has brought them repeated engagements in London’s West End and off-Broadway, as well as an ongoing tour. Side benefits included a regular BBC children’s channel hosting gig and another stage show in the works. It’s a long way from busking on the street.

“I’m almost his savior,” Clarkson says cheerfully about Turner.

On this day, they’re trying to enjoy the sights of Washington between evening performances there.

“We’re walking to the Smithsonian,” Clarkson says.

Taking the
phone, Turner scoffs at his partner’s poor memory. “We went to the Smithsonian yesterday,” he says. “We’re looking for another museum to go to today.”

But he doesn’t disagree with Clarkson’s account of their meeting: “I was trying to earn some money, retelling the story of ‘Treasure Island,’ if I remember correctly. It was the first time I’d ever tried it and the only time I’ve ever tried it, and I hope I’ll never busk again.”

And a strange guy came up and asked if you wanted to be Harry Potter?

“He offered me work, and I think any actor would go with somebody who offered them work,” Turner says.

The show could be a spoiler fest for someone who hasn’t read all the books yet, but Clarkson says not quite. “If you have read them, you’ll know what we’re talking about, but if you haven’t, then we don’t spoil the ending of the whole series for you.”

They’re not concerned that the last Potter novel was published in 2007, or the final movie premiered in 2011. Potter fandom will go on and on, Clarkson says.

He adds that they’re often in touch with Rowling’s cadre of lawyers and publicists, in large part to make sure their loving parody is considered fair use and not copyright infringement. “They actually came for a group outing to see us at Easter, which was very surreal,” he says.

There’s only been one reported contact with Rowling herself, “and I’d like to think it’s true,” he says.

As he tells it, they were at the Edinburgh Fringe for the first time when a box office worker, an 18-year-old girl, came to them in tears.

“And we said, ‘What’s wrong?’ And a manager came behind her and said, ‘A woman came to get a ticket to your show, and [the girl] turned her away because we were sold out.’ And we said, ‘Was she upset?’ And he said, ‘No, that woman was J.K. Rowling,’ ” Clarkson says and laughs.

“The box office girl hadn’t recognized her. But we now say that anywhere we are in the world — and this will also be the case in Boston — that we have one seat free so if [Rowling] happens to turn up and wants to see it, we’ve always got her a reserved seat.